1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method capable of resuming image printing operation so as to print an image continuous with a printed image after suspension of the printing operation.
2. Description of the Related Art
What are called serial scan type and full line type printing apparatuses are known as an inkjet printing apparatus in which a printing head capable of ejecting ink is used for image printing. The serial scan type is configured for image printing which involves movement of the printing head in a main scan direction and conveyance of a printing medium in a sub-scan direction intersecting the main scan direction, while the full line type is configured for image printing which involves the conveyance of the printing medium without involving the movement of the printing head.
Ink droplets ejected from the printing head include large major droplets, and satellites as tiny droplets. The major droplets are ejected at a predetermined rate, whereas the satellites ejected following after the major droplets are prone to be low in their rate of ejection and also unstable in their direction of ejection. Desirably, a facing distance between the printing head and the printing medium (hereinafter, sometimes called a “head-medium distance”) is reduced in order to reduce displacements of the landing positions of the ink droplets on the printing medium. The satellites, in particular, are susceptible to air resistance or flows of air, and therefore, a reduction in the head-medium distance is effective for a reduction in displacements of the landing positions of the satellites.
Setting the head-medium distance as described above requires taking into account the thickness of the printing medium, the floating of the printing medium above a printing medium conveyor line, and the expansion and contraction (hereinafter, sometimes called “cockling”) of the printing medium when it absorbs the ink.
The thickness of the printing medium can be determined beforehand prior to the printing operation, depending on the type of the printing medium. However, the cockling or the floating of the printing medium is difficult to determine before the printing operation, because the cockling or the floating depends on image conditions or environmental conditions during image printing. The cockling, in particular, is a phenomenon in which its size, range, or the like grows. Factors responsible for the growth of the cockling include the amount of ink ejected (or the amount of ink applied) to the printing medium, and the elapsed time since the time of image printing. A larger amount of ink ejected leads to a larger amount of ink absorbed into the printing medium and hence to a larger amount of expansion and contraction of the printing medium. In addition, a longer elapsed time since the time of image printing leads to an increasingly larger amount of ink absorbed into the printing medium, after which the ink is dried, and in turn, to a larger amount of expansion and contraction of the printing medium. It is difficult to maintain a certain head-medium distance, taking into account such a change in the cockling.
On the other hand, too short the head-medium distance may lead to contact of the printing head with the printing medium. Such contact may occur during reciprocating movements of the printing head in the main scan direction, for example in the serial scan type printing apparatus, particularly a consumer-oriented printing apparatus in which the printing medium is conveyed at a low rate of conveyance. In the full line type printing apparatus, the printing medium is continuously conveyed during printing operation, and thus, the printing medium which has cockled moves immediately past a printing position facing the printing head, which in turn reduces the likelihood of occurrence of the contact of the printing head with the printing medium. However, situations may arise where continuous printing operation involving the continuous conveyance of the printing medium is suspended for some cause and the printing of an image still in process of being printed is suspended and, thereafter, maintenance operation for the printing head or the like is performed and then the printing operation is resumed from a point in the image at which the printing has been suspended. In such situations, there is a possibility that the printing head contacts with the printing medium which has cockled before the suspension of the printing operation.
Japanese Patent Laid-Open No. 2009-119713 discloses a method which involves detecting the height of a printing medium varying by cockling, from a printed result of a cockling detection pattern, and setting the position of a printing head in its height direction under normal printing operation conditions, based on the detected height.
The method disclosed in Japanese Patent Laid-Open No. 2009-119713 can grasp beforehand the extent of cockling which may occur, based on the printed result of the cockling detection pattern. However, as described above, when printing operation for an image is suspended and thereafter the printing is resumed from a point in the image, a change in the cockling during the suspension of the printing operation cannot be grasped. At the time of such resumption of the printing, it is therefore necessary that the head-medium distance is set larger than an assumed value to avoid the contact of the printing head with the printing medium before it happens.